Reporter 458, 6 November 2000

Eclipse offers an opportunity

Chemists at Leeds have reported an experiment which took advantage of a solar eclipse to prove a point about the link between sunshine and smog.

They showed conclusively that a highly reactive chemical, which exists in the air for less than a second at a time but is essential in the control of atmospheric pollutants, disappears during darkness.

Researchers led by Dr Dwayne Heard and Professor Mike Pilling were in Berkshire on 11 August 1999 to make ground-level measurements of atmospheric changes in the open-air laboratory formed by the footprint of the near-total eclipse.

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The abrupt changeover from broad daylight to near-darkness and back again allowed highly accurate measurements to be made of hydroxyl radical (OH) levels.

The chemical is constantly produced in daylight conditions by ultraviolet light breaking down molecules, and it in turn breaks down pollutants like methane and carbon monoxide. Because reaction with OH is the only way these pollutants can be removed at ground level, OH is commonly referred to as the ‘detergent’ of the atmosphere. Its association with UV light means levels of OH are broadly proportional to the danger of sunburn.

The eclipse, causing a much faster rate of change in the intensity of the light than at sunset or sunrise, created the opportunity for very precise measurements to be made.

"As we had expected, OH levels plummeted during the eclipse. Levels of ozone, a component of smog formation, also dropped sharply when the sunshine was switched off", said Dr Heard.

The tests confirmed the importance of OH in ozone formation, and thereby the close relationship between sunshine and smog levels. They verified the basic understanding of atmospheric chemistry contained in computer models that inform current thinking on pollution control.

The researchers, who included Dr David Creasey, postgraduate James Lee and final-year undergraduate Joanne Abram, had the satisfaction of seeing their findings billed as the cover feature in the current issue of Geophysical Research Letters.

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